1. Field of the Invention
The present invention relates generally to controllers for lamps used to illuminate liquid crystal displays ("backlights") and the like and, in particular, to a method and apparatus for fast heating a cold cathode fluorescent lamp.
2. Description of the Related Art
Liquid crystal displays (LCD) provide a rugged and flexible display suitable for use in automotive applications. The LCD is backlit typically by a cold cathode fluorescent lamp (CCFL). Such fluorescent lamps are bright and relatively efficient and can be fabricated to provide even illumination over a large area. CCFL's are particularly useful to provide backlighting for illuminated vehicular displays.
Unfortunately, CCFL's are sensitive to temperature and vary in luminance as the passenger compartment and console warms up. During cold start conditions, for example, the initial luminance level of the CCFL may be unacceptably low to an operator of the vehicle. One method for compensating for this low luminance is to use a high-pressure self-heating type CCFL and to supply a "boost current" to the CCFL during startup. The boost current is an additional amount of lamp current above the normal maximum levels, resulting in an increased power supply, which is converted by the CCFL into heat to raise the lamp temperature, thereby facilitating increased lamp efficiency and a corresponding increased lamp luminance.
However, supplying a boost current increases the rate at which the mercury (Hg) inside the lamp is expended, causing premature failure resulting in extreme and sudden loss in luminance of the CCFL. For example, the life reduction of the CCFL operating at an ambient temperature greater than 30.degree. C. can be defined by the equation ##EQU1##
where L.sub.B is the life span of the CCFL using boost current, I.sub.B ; and L.sub.N is the normal CCFL life span under normal or recommended operating current, I.sub.N. The life of the CCFL under boost current is significantly reduced further when the ambient temperature is below 30.degree. C., as would be experienced during cold startup conditions. For example, it has been determined that the life span of the CCFL may be reduced by over 150 hours per start when the boost current is unnecessarily applied upon startup of the CCFL.
Accordingly, what is needed is a method and apparatus for supplying a boost current to a CCFL only when necessary during cold startup conditions.